My TV Chassis has developed a magnetic personality!
 

Referring back to my RCA 8PT, it seems that the chassis has developed a magnetic charge and is interfering with the brightness when set is reassembled.

After adjusting the ion trap for max brightness, the raster looks fine. Then, I'll put the CRT neck shield on and/or put the back portion of the chassis back together, over the CRT neck. When I power the set back up, however, the raster then becomes very dim.

I can reproduce this phenomena by passing its steel neck shield next to the neck while the set is on (the screen goes dim) or when I leave the neck shield off and pass the neck through the hole in the rear of the chassis (where it needs to be when set is reassembled.

I'm guessing the chassis and neck shield have become magnetized and are interfering with the ion trap magnetic field. Could this have happened because I often discharge the CRT to the chassis?

What does one do in this circumstance? Pass a degaussing wand or loop over the metal parts? Is there an alternative way to deal with this problem? I don't currently have degaussing equipment.

I heard passing a running drill motor over the parts may degauss, but I have no idea.

Thanks in advance...

I don't think discharging the anode to the chassis will magnetize anything, but any steel components like the chassis and brackets can pick up stray magnetism on their own.

With color CRTs, the brackets that hold the tube down are just as culpable for purity errors as the shadow mask is.

I've never degaussed a B&W TV but I would in your case given your symptoms.

You can make a degausser or buy one, but I don't think a small magnetic tape degausser or electric motor tool can reliably and cleanly demagnetize a large area.



John

I don't know how much effect a degaussing would have on the ion trap, but to be on the safe side, I would remove the ion trap before I degaussed the chassis. If it has centering rings behind the yoke, I'd remove them as well.

I'm not sure there'd be any significant effect on those. Back when the DC adjustments on color TV convergence were actually magnets, and the purity and blue lateral adjustments were magnetic rings, there was never an admonition from the manufacturers about protecting those items during degauss. I also never saw any change in those adjustments after a thorough degaussing.

My gut feeling is that they would be safe from a typical service degaussing coil.

John

If you have old focus coil from a early post war set popular mechanics had articles on how to make one into a compound tool magnetizer/demagnetizer. If you have a junk BPC TV I posted a sticky in the TV and radio tech forum on how to make one into a service duty degaussing coil out of one.

If you have a bulk magnetic tape eraser that might work.

Right now, I'm using a Radio Shack bulk tape eraser.
I have a regular degaussing coil I've since 1966, when I first starting repairing color sets, but it's in my warehouse some where.

The Ion Trap is supposed to be adjusted after the set is together.

Originally there was a piece of stiff paper (possibly Fishpaper) attached to the Ion Trap and long enough to reach through the hole in the back of the cabinet. Many of them have long since rotted off and disappeared.

I encountered this problem on the very first one I did, drove me crazy until I figured it out.

Hi Members.i have heard of using a soldering gun to degauss a picture tube in a pinch.never tested since I have had a degaussing coil since I was a kid.

Yes.

After a bit of back and forth, I came to this conclusion as well. That, the trap needs to be adjusted AFTER the set is reassembled. The trap is simply designed to work in this configuration. In this case it's a royal pain because of the surrounding neck shield and how difficult it is to maneuver, working in such a narrow tube. I have to see if I can figure out the best tool to use. So far a needle nose pliers or long flat blade screwdriver have been the best choices but very difficult to use. You can push the magnet in but moving it back out or side to side is nearly impossible.

Also wondering if I should replace the original thick fish paper with new stuff that is thinner and allows for easier adjusting of the ion trap...

Thin Fish paper should work, it doesn't take much to move it around.
There's too much risk of damaging the tube using anything metal. A squirt of Silicon Spray may make it easier to move side to side.

I wonder how much effect the power transformer has with it located so closely to the ion trap.

Under normal circumstances, there is no DC on the transformer.. :banana:

John

This seems to be a common issue with the 8-PT. I'm glad I'm not the only one having it!

If any of you have read up on Phil Nelson's 8-PT restoration, he mentions issues with magnetism in the cabinet. In his case it pulled the picture at a slight angle down to the side the power transformer was on. Here's that link: https://antiqueradio.org/rca16.htm

A few years ago, I actually bought two of his 8-PTs off of him, including the set mentioned in that link. I actually swapped cabinets to use the restored chassis in a cabinet with legs (don't worry, nothing was thrown out. If I ever get some time together I'm hoping to restore the other chassis and swap cabinets back), and encountered the exact same problem on mine despite being in a new cabinet! No matter how far the yoke is shifted the opposite direction, it always angles down the exact same way. It has to be something do with the chassis layout.